In general, a mobile communication system, which is utilized in wireless data communications, personal communication services, wireless local networks, etc., includes a base station, at least one relay system, and subscriber terminals; and the relay system includes a link antenna, a repeater, and a coverage antenna.
The link antenna receives a signal from the base station and transmits the signal to the repeater, and the repeater relays signals between the link antenna and the coverage antenna. Furthermore, the coverage antenna transmits the signal received through the link antenna and the repeater from the base station to a subscriber terminal and transmits a signal received from a subscriber terminal to the repeater.
In the relay system operating as described above, the coverage antenna is located at an opposite position of the link antenna while being spaced a predetermined distance from the link antenna, so that sufficient antenna isolation is secured between the coverage and link antennas, that is, the transmission and reception antennas.
In this case, the antenna isolation signifies the difference between intensities of a signal received by the link antenna from the base station and a signal fed back to the link antenna after being amplified in the repeater and then emitted through the coverage antenna.
When the difference between the intensities of the two signals is small, the signal received by the link antenna generates oscillation of the repeater, and repetitive oscillation of the repeater may cause the repeater to go out of order.
Therefore, in order to prevent the repeater from oscillating, the signal-reception level of the base station must be at least 10 dB higher than the signal-reception level of the repeater.
Conventionally, three methods as described below are usually employed in order to secure proper antenna isolation.
First, a high gain directional antenna having a high front-to-back ratio is used.
Second, distance between the coverage antenna and a donor of the repeater is increased.
Third, antennas are shielded from each other.
In the first method, a channel selector repeater, a microwave repeater, a laser repeater, or an optical repeater may be utilized. However, such repeaters are expensive, and it is difficult to repair and maintain such repeaters.
In the second and third methods, a line as long as an increased physical distance between the antennas must be added, which thereby causes power loss between the repeater and antennas.
Further, the performance of the relay system is deteriorated, thereby reducing service coverage.
Moreover, the three conventional methods are disadvantageous in view of installation and maintenance of equipment, in that it is impossible to employ these three conventional methods in a restricted space, and relatively large expense is required in employing these three conventional methods.